Guide
Why your boiler keeps losing pressure
How to re-pressurise a combi or system boiler safely, the usual reasons pressure drops, and when it needs an engineer.
Updated
What the pressure should be
Combi and system boilers run on a sealed system with a pressure gauge on the front or underneath. With the heating off and the system cold, the needle should usually sit between about 1 and 1.5 bar. It rises a little when the heating is on, which is normal. Check your boiler manual for its exact range.
Below about 0.5 to 1 bar, most boilers stop firing and show a low pressure fault. Above about 3 bar, a safety valve opens to release water, which is a sign of a different problem.
- Below 1: top up
- 1 to 1.5: right, when cold
- Near 3: too high
How to top it up
Most boilers have a filling loop: a braided hose or a built-in key or lever under the boiler with a valve at each end. With the system cold, open the valves slowly and watch the gauge, close them when it reaches about 1 to 1.5 bar, then reset the boiler. The manual will show the exact method for your model.
Re-pressurising is something a householder can do. It does not involve the gas side of the boiler. Never open the boiler’s case: that is Gas Safe work.
Why it keeps dropping
If you have to top up more than once or twice a year, water is leaving the system somewhere. The usual suspects are a weeping radiator valve or pipe joint (look for damp patches, stains or green corrosion), a pressure relief valve letting by (look for a copper pipe through an outside wall that drips), or a failing expansion vessel inside the boiler, which also makes pressure swing high when the heating is on.
Bleeding radiators lets some water out and lowers the pressure, which is expected. Finding the real leak needs an engineer if it is not visible. Ignoring it means the boiler keeps locking out, and a hidden leak can damage floors and ceilings.